Method for the Synchronization of a Radio Receiver, and Adapted Receiver for the Implementation of Said Method

ABSTRACT

A method for the synchronization of a radio receiver, comprising an estimation of the moment when a pulse ( 11, 17 ) is received ( 11, 17 ), performed from the moment when a previous pulse was received. The estimated moment is compared with the real moment when the pulse ( 21, 27 ) is received in order to validate an association of pulses with values of a code recorded in the receiver ( 31, 37 ). A moment for the beginning of transmission of a symbol is thus deduced, enabling the receiver to be synchronized in relation to the transmitted radio pulse sequence.

The present invention relates to a method of synchronizing a radioreceiver, and a receiver suitable for implementing such a method. Italso relates to a transmission method comprising such a synchronizationof the receiver, and a suitable radio transmitter.

Pulse radio transmission is a recent remote communication technique,used mainly for short range applications. The transmission distance istypically less than 100 meters. It can cover transmissions at very highrate, at least 500 megabits/second for example, used in variouscircumstances such as, in particular, a USB2 port, a video datatransmission between a DVD player and a projector, or even a databroadcast from an information station made available to the public. Itcan also cover transmissions at relatively low rate, for example forlinking sensors to a monitoring or data acquisition local area network.In this case, the pulse transmission is particularly advantageousbecause of its low energy consumption.

In a pulse radio transmission, a predetermined transmission duration isassigned to each symbol. The term “symbol” is used to mean a modulationstate used in coding data intended for transmission. For a 2-PPM(standing for “Pulse Position Modulation”) transmission mode, a symbolcorresponds to one transmitted bit, and for a 4-PPM mode, a symbolcorresponds to two transmitted bits. The symbol transmission duration isdivided into a fixed number of first-level time slots, called frames.Furthermore, each frame is itself divided into second-level time slots,called slots. As an example, the transmission duration of a symbol(denoted T_(S)) is 1280 ns (ns standing for nanoseconds), the durationof a frame (denoted T_(F)) is 160 ns and that of a slot (denoted T_(C),standing for “chip” duration) is 20 ns. In this case, one symbolcorresponds to eight frames transmitted in succession, and each frame inturn corresponds to eight slots.

A symbol is transmitted in the form of pulses located within certainslots of a transmission sequence. It is coded via a delay of each pulserelative to the start of the corresponding slot, called state delay. Toresume the example mentioned above, a pulse duration of around 1 ns anda state delay also of around 1 ns are suitable for 20 ns slots. Becauseof the very short duration of the pulses, the transmitted radio energyis distributed over a very wide range of frequencies, of the order ofseveral gigahertz. For this reason, the pulse radio transmission mode isdesignated UWB, standing for “Ultra-Wide Band”.

Within the transmission sequence of each symbol, different slotselections for each frame make it possible to obtain multiple access. Inother words, several transmission channels are thus distinguished, whichcan be used simultaneously to transmit a variety of data to differentreceivers. The ordered series of position numbers, within successiveframes, of the slots that contain a meaningful pulse to transmit asymbol via this channel constitutes a characteristic code of thechannel. This code is therefore formed by a pseudo-random series of Nvalues, N being the number of frames within the transmission sequence ofa symbol. Each value is an integer number between 1 and the number ofslots per frame. FIG. 1 illustrates a transmission sequencecorresponding to one and the same symbol transmitted simultaneously bytwo separate channels. In the case of the example illustrated, the codesused for each channel are respectively 1, 3, . . . , 7 and 5, 4, . . . ,2.

Changing slot between two successive frames for one and the samechannel, according to the code of the latter, is called time hopping. Inaddition to the multiplexing function which has been described, timehopping makes it possible to introduce redundancy, in order to improvethe transmission quality. This redundancy corresponds to themultiplicity of the frames indicated by the code and which contain aparticular symbol.

Furthermore, time hopping makes it possible to break up the periodicityof the transmitted radio signal. The energy of the radio signal is thendistributed more continuously over all the frequency spectrum.

To identify a symbol transmitted by a transmitter to a predeterminedreceiver, the receiver must be synchronized relative to the transmitter.Once the synchronization is obtained, a symbol correlation pattern isgenerated within the receiver at the same time as the radio signal isreceived. This pattern is generated from the code of the channel used,and makes it possible to isolate, within the received signal, the pulsestransmitted to the receiver. For this, the synchronization of thereceiver must be performed with an accuracy of a few tens ofpicoseconds, typically.

Usually, the synchronization of the receiver is sought by implementing arolling correlation between the pattern and the received radio signal.For this, correlation ratios between a sequence of the received signaland the pattern are calculated, by applying variable delay values to thepattern. The delay value associated with the highest correlation ratioobtained in this way corresponds to the synchronization. Given that theaccuracy of the synchronization is then proportional to the number ofdelay values for which a correlation ratio is calculated, a considerablenumber of operations must be carried out within the receiver.

Such a synchronization method can therefore take a particularly longtime, especially for low-rate transmissions. Now, the data that iscontained in the signals received before the receiver is synchronizedcannot be decoded. The result of this is that a large quantity oftransmitted useful data is lost when setting up a new pulse radioconnection.

Furthermore, the method of synchronization by rolling correlationrequires the receiver to have major calculation means. In particular,several computer assemblies are usually arranged in parallel within thereceiver, to reduce the time it takes to search for synchronization. Theresult is a complexity, a production cost and an energy consumption ofthe receiver that are not compatible with certain uses considered forpulse radio transmission.

Various refinements of synchronization by rolling correlation have beenrecently introduced, in order to reduce the duration of this operation.These refinements provide a faster convergence towards the value of thedelay applied to the pattern which corresponds to the maximum of thecorrelation ratio. Despite these refinements, the synchronization of thereceiver remains a lengthy and detrimental step.

Finally, synchronization by rolling correlation is incompatible with areceiver operating by energy detection. In practice, the amplitude ofthe radio signal must be identified by the receiver at each moment, evenfor very low signal levels, in order to calculate the correlation ratiosbetween this amplitude and the correlation pattern.

One aim of the present invention is therefore, in the context of pulseradio transmission, to synchronize a receiver quickly and withoutrequiring major calculation means within the receiver.

To this end, the invention proposes a method of synchronizing a radioreceiver suitable for receiving coded symbols in the form of radiopulses, each transmitted in a predetermined time slot within arespective frame, said slot being determined by a value selected from aset of ordered values forming a code stored in the receiver, the methodcomprising the following steps, performed within the receiver:

-   -   a—detecting received pulses;    -   b—storing moments of reception of said pulses;    -   c—associating a first value selected from the code with a first        received pulse;    -   d—selecting a second value from the code;    -   e—estimating at least one moment of reception by combining the        moment of reception stored for said first received pulse with        said first and second selected code values;    -   f—if the estimated moment matches the moment of reception stored        for a second received pulse, validating the associations of said        first and second code values with said first and second received        pulses,        -   otherwise repeating steps c to f by varying at least one of            said first and second selected code values; and    -   g—deducing, from at least one of the values of the code whose        selection has been validated, a moment of transmission start for        a symbol.

Thus, in a synchronization method according to the invention, thequantity of calculations performed within the receiver is not a priorifixed, but the calculations are continued until the synchronization isfound. In other words, predefined calculation sequences are no longercarried out systematically, but only a minimal quantity of calculationsis carried out, which is adapted according to the received radio signal.The result is a significant reduction in the average calculation timeneeded to synchronize the receiver relative to the radio signal. Farless of the transmitted useful data is then lost while setting up a newradio connection.

The reduction in the average quantity of calculations needed tosynchronize the receiver also makes it possible to simplify thecomputation means with which the receiver is equipped. Less complex andmore lightweight receivers, having a lower energy consumption, cantherefore be used.

One method of synchronization according to the invention is compatiblewith operation of the receiver by energy detection. Step a can thencomprise the following substeps:

-   -   a1—extracting an envelope of a received radio signal;    -   a2—comparing an amplitude of the extracted envelope with a        detection threshold stored in the receiver; and    -   a3—if said amplitude exceeds the detection threshold,        identifying the radio signal with a received pulse.

Circuit modules already developed to perform such radio detectionoperations can then be used again to equip a receiver suited to theinvention. Such re-use of existing circuit modules helps to reduce thedesign cost of the receiver.

According to a first improvement of the invention, the detectionthreshold can be adjusted so as to detect, during a duration oftransmission of two successive symbols, at least as many pulses as apredetermined fraction of the number of values contained in the code.This fraction can be set, for example, at half the number of valuescontained in the code. Enough pulses are thus detected by the receiver,for which a match can be verified with moments of reception estimatedfrom code values. The possibility of a pulse that is not detected by thereceiver preventing the synchronization from being achieved is thenreduced. In the jargon of those skilled in the art, the synchronizationmethod is said to be more “robust”.

According to a second improvement of the invention, a series of matchesis sought, between the moments estimated from the code on the one hand,and the moments of reception of at least three pulses on the other hand.The synchronization that is found is then confirmed by several matches,so that the risk of resulting in an erroneous synchronization because ofchance coincidences or noise pulses is reduced. The synchronizationobtained is therefore particularly reliable. For this, the method alsocomprises the following steps, executed after the selections of m codevalues have been validated, m being an integer number greater than orequal to 2:

-   -   d′—selecting an (m+1)th value from the code;    -   e′—estimating at least one moment of reception by combining the        moment of reception stored for the mth received pulse with the        mth and (m+1)th selected code values; and    -   f′—if the estimated moment matches the moment of reception        stored for an (m+1)th received pulse, validating the association        of the (m+1)th code value with the (m+1)th received pulse,        -   otherwise repeating steps d′ to f′ by varying the (m+1)th            value selected of the code.

The method can be continued recurrently to obtain series of selected andvalidated code values. It is stopped when enough code values have beenassociated with received pulses. The synchronization of the receiver isthen determined with sufficient certainty. For example, the method canbe continued until a series of validations is obtained that correspondsto at least half the values contained in the code.

The invention also proposes a radio receiver suitable for implementing asynchronization method as described previously.

It also proposes a radio transmission method comprising the followingsteps:

-   -   coding of the symbols in the form of pulses distributed within        respective frames, each pulse being transmitted in a slot of the        corresponding frame determined by a value of a code stored in a        radio transmitter and in at least one radio receiver;    -   transmission of the pulses by radio channel; and    -   synchronization of at least one receiver by using a method as        described previously.

The invention also proposes a radio transmitter suitable forimplementing such a transmission method.

It also proposes a system of radio transmission by pulses comprising atransmitter and a receiver as described previously.

It finally proposes a synchronization device for a radio receiver thatis suitable for receiving symbols coded in the form of radio pulses,each transmitted in a predetermined time slot within a respective frame,this slot being determined by a value selected from a set of orderedvalues forming a code stored in the receiver. Such a synchronizationdevice comprises:

-   -   a—means for storing moments of reception of pulses;    -   b—means for selecting values from the code;    -   c—means for calculating at least one second moment for reception        from first and second selected code values and a first moment of        reception stored for a first received pulse;    -   d—means for validating associations of first and second selected        code values with first and second received pulses, organized so        as to be activated if a second calculated moment of reception        matches the moment of reception stored for the second received        pulse; and    -   e—means for determining a moment of transmission start for a        symbol, suitable for determining said start moment from at least        one of the code values whose selection has been validated.

Other features and advantages of the present invention will becomeapparent from the description that follows of an exemplary andnon-limiting implementation, with reference to the appended drawings, inwhich:

FIG. 1, already described, illustrates the structure of a pulse radiotransmission sequence;

FIGS. 2 a and 2 b, intended to be associated, represent a block diagramof a synchronization method according to the invention; and

FIG. 3 represents a tree diagram of the execution of certain steps of asynchronization method according to the invention.

The invention will now be described in detail for a pulse radiotransmission mode according to FIG. 1. The successive values of the codeare denoted c(i), with i=1, 2, . . . , 8. Consequently, for the firstexample considered in FIG. 1: c(1)=1, c(2)=3, c(8)=7. Furthermore, inthis example, the number of pulses used to code a symbol is equal to thelength of the code.

The synchronization method comprises two tasks executed in parallel. Thefirst task consists in detecting pulses in the received radio signal,and in storing the moment of reception of each pulse detected. Itcorresponds to steps a and b introduced above. The second task consistsin finding matches between moments of reception estimated from a firstreceived pulse, and moments at which other pulses are receivedsubsequently. This second task corresponds to generic steps c to f andd′ to f′.

In order to obtain a synchronization of the receiver more quickly, it isadvantageous to break down the second task into two separate phases. Thefirst phase corresponds to the estimating of the moments of reception ofpulses from the values of the code, and the second phase corresponds tocomparing the estimated moments of reception and the real moments ofreception of pulses. The first phase can be executed by a processor ofthe receiver without waiting for the pulses used for the comparison tobe received. The second phase can then be executed as and when thepulses are received. For this, one possible implementation of thesynchronization method consists in having steps c to g executed asbackground tasks by the processor of the receiver. The step b of themethod is then repeated during interruptions of the execution of thebackground task, the interruptions being triggered in response to thedetection of new pulses. Thus, the first phase of the second task, whichcan take a certain calculation and stored data manipulation time, can bestarted and continued independently of the state of progress of thefirst task. Only the execution of the second phase of the second taskdepends on the progress of the first task, that is, the frequency withwhich pulses are detected by the receiver. The time between thereception of the first detected pulse and the moment whensynchronization is obtained is thus greatly reduced in most cases.

The first task of the synchronization method will now be described inconnection with FIG. 2 a. In a way that is already implemented inexisting appliances, the radio signal is received by a detection unit ofthe receiver, and an envelope of the signal is extracted by carrierelimination (step 1). The amplitude of the envelope is then comparedwith a fixed detection threshold (step 2). Each new violation of thethreshold, which occurs after a decrease in the amplitude of theenvelope below the threshold, is interpreted as a new pulse received.

The moment of reception of the received pulse can be determined simply.For example, a rising edge of a specific electronic signal can betriggered for each pulse received. The moment of reception of the pulsethen corresponds to the moment at which the rising edge is triggered. Itis identified relative to a clock signal internal to the receiver. Othermethods of determining the moment of reception of a pulse canalternatively be used, provided that they provide an accuracy compatiblewith the pulse radio transmission method.

The actual moments of reception of pulses, denoted t₁, t₂, t₃, . . . ,are stored within the receiver, in a dedicated memory (steps 4-6). Sucha record of the moments of reception is advantageously presented in theform of a chronologically ordered list, complemented each time a newpulse is detected. The first task, which culminates in the storage ofthe real moments of reception of pulses, is continued until thesynchronization is definitively determined. As will be explained later,it may be advantageous to vary the pulse detection threshold used in thefirst task (step 3).

The second task proceeds according to a tree, as represented in FIG. 3.Each branch of the tree, or node, corresponds to an attempt to associatea value c(1), c(2), . . . , c(8) of the code with an additional receivedpulse. Such a branch constitutes an extension of the tree in ahorizontal dimension. Furthermore, associations with values of the codetried successively for different received pulses form branches of thetree that extend in a downward direction. The path of a branch thereforecorresponds to the recognition of additional pulses. Each pulse receivedand recognized forms a level of the tree, at which the nodes arelocated. Each level is identified on the left of FIG. 3 by the momentt₁, t₂, t₃, . . . of reception of the corresponding pulse. The size ofthe tree, in both horizontal and downward directions, continues toincrease until the synchronization has been determined. However, thesize of the tree in the downward direction is limited by the number ofslots per frame, and the size of the tree in the horizontal direction islimited by the length of the code. If the synchronization is determinedwith the first associations tried between code values and receivedpulses, the tree that is constructed is small. The synchronization ofthe receiver is then very fast.

For each pulse received, the code values are tried in turn, for examplein the order of the values in the code, until one of them can beassociated with the pulse. Preferably, the first code value tried for areceived pulse is the one that follows, in the order of the code, thevalue successfully associated with the preceding pulse for the samebranch of the tree. When all the code values up to the last, that is upto c (8) in the example, have been tried unsuccessfully for a pulse, theassociation tests for this pulse are continued with c(1), and then withthe successive code values until the one that in the code precedes thevalue of the node located just above in the same branch of the tree isfound.

The second task begins by simultaneously considering the first twopulses received, respectively numbered 1 and 2. The attempts toassociate these two pulses with two code values constitute the first twolevels of the tree of FIG. 3. The first level of the tree, correspondingto t₁, combines all the code values worked through from c(1). The secondlevel of the tree, which corresponds to t₂, combines, for each value ofthe first level, all the code values other than that indicated in thefirst level for the same branch. All the branches of the tree are thusinitiated.

Associating the pulses 1 and 2 with any two code values c(j) and c(k), jand k being two integer numbers between 1 and the number of code values,consists in looking for a match between t₂ and the moment of receptionof a second pulse estimated from t₁ as follows:if k>j:t _(estimated)(2)=t ₁ +[c(k)−c(j)]×T _(c)  (1)if k<j:t _(estimated)(2)=t1+T _(F) +[c(k)−c(j)]×T _(c)  (2)

Various match criteria between t_(estimated)(2) and t₂ can be adopted.The inventors have observed that an appropriate criterion involveschecking that the absolute value of the difference betweent_(estimated)(2) and t₂ is less than T_(c)/4. As a general rule, thiscriterion can be stated as follows: in step f, the moment of receptionestimated for a second pulse from the moment of reception stored for thefirst pulse matches the moment of reception stored for said second pulseif an absolute value of a difference between the moments of receptionestimated and stored for the second pulse is less than a predeterminedfraction of the duration of a time slot. This match threshold can beequal to T_(c)/4 or T_(c)/16, for example.

Back to FIG. 2 a, the steps referenced 11, 12, . . . , 17 correspond tothe calculation of t_(estimated)(2) by associating c(1) with the pulse1. For clarity in FIG. 2 a, Δ _(jk) is used to designate the combinationof c(j) and c(k) which is added to the moment of reception stored for apulse in order to obtain the moment of reception estimated for asubsequent pulse. Thus:if k>j:Δ _(jk) =[c(k)−c(j)]×T _(c)  (3)if k<j:Δ _(jk) =T _(F) +[c(k)−c(j)]×T _(c)  (4)for the association of the pulse 1 with c(1), that is j=1, and for eachpossibility of associating the pulse 2 with one of the code values otherthan c(1), the match between t_(estimated)(2) and t₂ is evaluated in thesteps referenced 21, 22, . . . , 27. When a match is obtained in one ofthe steps 21, 22, . . . , 27, the association of the pulse 1 with c(1)and of the pulse 2 with the corresponding value c(k) is validated(respective steps 31, 32, . . . , 37). Otherwise, steps 11-17 arerepeated, starting from the node c(2) of the first level of the tree(FIG. 3), and so on, working through all of the first and second levelsof the tree.

The first and second levels of the tree of FIG. 3 correspond to thegeneric steps c and d introduced above. Steps 11, 12, . . . , 17correspond to the generic step e, for each combination of first andsecond code values. Similarly, steps 21, 22, . . . , 27 and 31, 32, 37correspond to the generic step f.

If no match is found for the received pulses 1 and 2 with a pair of codevalues c(j), c(k), the pulse 1 is considered to belong to the whitenoise and is discarded. For this, the moment t₁ is erased from thememory of the receiver. The received pulses 2, 3, . . . are thenrespectively renumbered 1, 2, . . . (step 28) and the second task of themethod is restarted from the new pulse 1.

Immediately a validation has been obtained in one of the steps 31-37 fora pair of values c(j), c(k), the synchronization of the receiver isdetermined by calculating the moment t₀ of the start of transmission ofa symbol. t₀ is then given by the following formula:t ₀ =t ₁ −c(j)×T _(c)−(j−1)×T _(F) +T _(S)  (5)

The execution of steps 11-17 and 21-27 for different pairs of codevalues can be stopped immediately a moment to has thus been computed.Alternatively, in the case where a synchronization established over morethan two pulses is sought, the execution of the generic steps c to f canbe continued according to the first two levels of the tree. Validationsmay, possibly, thus be obtained for several different branches of thetree.

Since the steps 11-17 are independent of t₂, they can be executed beforethe pulse 2 has been detected by the receiver. In particular, thegeneric steps c to e can be repeated for several different pairs offirst and second code values selected before the second pulse isdetected. Step f is then executed for at least one of said pairs offirst and second values after the second pulse has been detected.Obtaining a synchronization of the receiver is consequently delayedminimally by a possible delay in reception of the second pulse after thefirst pulse.

One method of synchronization based on just two pulses is finishedimmediately a validation has been obtained according to the generic stepf. The description that follows concerns a continuation of the methodwhen the synchronization is sought over more than two pulses.

Moments of arrival of a third pulse are then estimated from t₂ asfollows (steps 41 and 42 of FIG. 2 b)t _(estimated)(3)=t ₂+Δ_(mn)  (6)

In this relation, m is the number of the code value c(m) for which theassociation of the pulse 2 has been validated for the same branch of thetree. n is the number of the code value c(n) for which the associationwith the moment t₃ of reception of the third pulse is tried. Δ_(mn) isobtained from the formulae (3) and (4) by replacing j and k respectivelywith m and n.

A match between t_(estimated)(3) and t₃ is sought in the step 51 of FIG.3, for m=2 and n=3. The match criterion used already and described abovecan be re-used. In case of a match, the association of c(3) with thethird pulse is validated (step 61), which corresponds to the pathdenoted P1 in the tree of FIG. 3. If t_(estimated)(3) does not match t₃,the association of c(4) with the third pulse is then tried (steps 42, 52and 62). The search for a code value that can be associated with thethird pulse received is then continued by working through the portion E1of the third level of the tree (FIG. 3). Thus, the generic steps d′ tof′ can be repeated for several different code values selected as thirdvalue following the first and second values. If necessary, steps d′ ande′, which do not require a knowledge of t₃, can be repeated for severaldifferent code values selected as third value before a third pulse isdetected. Step f′ is then executed for at least one of the valuesselected as third value after the third pulse has been received. In thiscase, any delay in the execution of the first task of the methodgenerates only a limited delay to obtain the synchronization of thereceiver, since the steps of the second task of the method arenevertheless executed during this delay.

If the third pulse cannot be associated with any code value, it isdiscarded and replaced with the next pulse received (step 58).

Continuing the method, a code value is associated with each pulsereceived that is not ruled out. Nevertheless, it is possible that nopulse received can thus be associated with a given value of the code.According to one refinement of the invention, the generic steps d′ to f′are then repeated by selecting a code value for which no association hasbeen validated, and by searching for a match between the estimatedmoment of reception and a stored moment of reception of a pulse advancedby a whole number of symbol transmission durations. For this, steps 41,42, . . . , 51, 52, . . . are repeated for the values of the code thathave not yet been associated with a pulse, by introducing at least onesymbol transmission duration into the match criterion. The two pulsesthat occur in these steps can therefore belong to different symboltransmission sequences, and the match between the estimated and realmoments of reception is sought by subtracting from the actual moment ofreception at least one times the duration T_(s) (step 59). Such acontinuation of the method over a transmission duration covering severalsymbols can make it possible to extend certain branches of the tree ofFIG. 3.

In order to increase the probability of detecting a pulse that can beassociated with each code value, the detection threshold used in thestep 2 (FIG. 2 a) can even be adjusted throughout the method,particularly if a large number of code values have not yet beenassociated with pulses. The inventors have observed that, as a generalrule, the number of pulses detected does not increase progressively whenthe detection threshold is lowered. On the contrary, several pulsesoften exhibit roughly identical amplitude levels, such that they aredetected below one and the same threshold value. According to oneinterpretation, pulses having roughly identical amplitude levels followthe same propagation path between the transmitter and the receiver. Theattenuation level of the radio signal then depends on the propagationpath followed, and several different propagation paths are followedsimultaneously. Advantageously, the detection threshold can be loweredon each adjustment so as to take into account an additional propagationpath. Thus, the threshold of detection of the pulses can be lowered bysuccessive steps so as to detect, during a symbol transmission duration,at least as many additional pulses in each lowering step as the averagenumber of pulses used to transmit a symbol.

The count of the associations validated according to one branch of thetree is updated each time an additional association is validated forthis branch. Where appropriate, this count can take into account onlythe validations relating to different values of the code. Thesynchronization method is continued until the number of validatedassociations of one and the same branch of the tree confers an adequatelikelihood on these associations (steps 71). The moment to of the startof transmission of a symbol is then computed by using the formula (5)for this branch of the tree (step 72). The synchronization method isthen finished and the receiver undertakes to decode the receivedsymbols.

As an example, the likelihood criterion can be that the number ofvalidated selections of different code values according to one and thesame branch of the tree is greater than half the number of valuescontained in the code.

When the synchronization of the receiver is sought based on pulses usedto code symbols, it is possible for certain frames not to include pulsesfor several successive symbol transmission sequences. Consequently, thecode values corresponding to these unused frames cannot be associatedwith pulses. To avoid such circumstances leading to the synchronizationmethod being excessively prolonged, it is advantageous to provide apreamble for each transmitted data packet, dedicated to thesynchronization of the receiver, and which contains one pulse for eachframe. Such a preamble makes it possible to reduce the synchronizationdelay on setting up the connection. It may be at least as long as atransmission sequence of two successive symbols, for example. In thiscase, at least some of the radio pulses used to synchronize the receiverbelong to the preamble of a data packet. Where appropriate, the pulsesof the preamble all correspond to one and the same state delay, measuredrelative to the start of the slot containing each pulse. Thesynchronization accuracy is then greater, given that no state delayoccurs randomly when determining the moments of reception of the pulses.

It is understood that numerous modifications and adaptations can beintroduced into the synchronization method that has been described indetail above. As an example, the inventors cite the following possibleadaptations:

-   -   the number of pulses used to code a symbol may differ from the        length of the code;    -   the code values used to identify a channel can be any        alphanumeric symbols;    -   the storage of the moments of reception of pulses can be limited        to the last two pulses received, by updating them each time a        new pulse is received;    -   the storage of the moments of reception of the pulses can be        replaced by a storage of time differences separating successive        pulses; and    -   steps c to e can be carried out only after the reception of the        second pulse, in the same way that the steps d′ and e′ can be        performed only after the reception of the (m+1)th pulse. In this        case, the calculations needed to associate a new pulse with a        code value are executed in response to the reception of the        pulse.

1: A method of synchronizing a radio receiver suitable for receivingcoded symbols in the form of radio pulses, each transmitted in apredetermined time slot within a respective frame, said slot beingdetermined by a value selected from a set of ordered values forming acode stored in the receiver, the method comprising the following steps,performed within the receiver: a—detecting received pulses, b—storingmoments of reception of said pulses; c—associating a first valueselected from the code with a first received pulse; d—selecting a secondvalue from the code; e—estimating at least one moment of reception bycombining the moment of reception stored for said first received pulsewith said first and second selected code values; f—if the estimatedmoment matches the moment of reception stored for a second receivedpulse, validating the associations of said first and second code valueswith said first and second received pulses, otherwise repeating steps cto f by varying at least one of said first and second selected codevalues; and g—deducing, from at least one of the values of the codewhose selection has been validated, a moment of transmission start for asymbol. 2: The method as claimed in claim 1, wherein steps c to g areexecuted as a background task by a processor of the receiver, andwherein step b is repeated during interruptions of the of the backgroundtask, said interruptions being triggered in response to the detection ofnew pulses. 3: The method as claimed in claim 1, wherein at least someof the radio pulses used to synchronize the receiver belong to a datapacket preamble, said preamble having one pulse for each frame. 4: Themethod as claimed in claim 3, wherein the preamble is at least as longas a sequence of transmission of two successive symbols. 5: The methodas claimed in claim 3, wherein the pulses of the preamble all correspondto one and the same state delay, measured relative to the start of thetime slot containing each pulse. 6: The method as claimed in claim 1,wherein step a comprises the following substeps: a1—extracting anenvelope of a received radio signal; a2—comparing an amplitude of theextracted envelope with a detection threshold stored in the receiver;and a3—if said amplitude exceeds the detection threshold, identifyingthe radio signal with a received pulse. 7: The method as claimed inclaim 6, wherein a rising edge of an electronic signal is triggered foreach received pulse, and wherein the reception moment of the pulsecorresponds to the moment of the triggering of the rising edge. 8: Themethod as claimed in claim 6, wherein the detection threshold isadjusted so as to detect, during a duration of transmission of twosuccessive symbols, at least as many pulses as a predetermined fractionof the number of values contained in the code. 9: The method as claimedin claim 8, wherein the detection threshold is lowered in successivesteps so as to detect, during a duration of transmission of a symbol, atleast as many additional pulses in each detection threshold loweringstep as the average number of pulses used to transmit a symbol. 10: Themethod as claimed in claim 1, wherein, in step f, the moment ofreception estimated for a second pulse from the moment of receptionstored for the first pulse matches the moment of reception stored forsaid second pulse if an absolute value of a difference between theestimated and stored moments of reception for the second pulse is lessthan a predetermined fraction of the duration of a time slot. 11: Themethod as claimed in claim 1, wherein steps c to e are repeated forseveral different pairs of first and second code values selected beforethe second pulse is detected 3, and wherein step f is executed for atleast one of said pairs of first and second code values selected afterthe second pulse has been detected. 12: The method as claimed in claim1, also comprising the following steps, executed after the selections ofm code values have been validated, m being an integer number greaterthan or equal to 2: d′—selecting an (m+1)th value from the code;e′—estimating at least one moment of reception by combining the momentof reception stored for the mth received pulse with the mth and (m+1)thselected code values (11, 42, . . . ); and f—if the estimated momentmatches the moment of reception stored for an (m+1)th received pulse,validating the association of the (m+1)th code value with the (m+1)threceived pulse, otherwise repeating steps d′ to f by varying the (m+1)thvalue selected of the code. 13: The method as claimed in claim 12,wherein steps d′ and e′ are repeated for several different code valuesselected as (m+1)th value before the (m+1)th pulse is detected (6), andwherein step f′ is executed for at least one of the values selected as(m+1)th value after the (m+1)th pulse has been received. 14: The methodas claimed in claim 12, wherein steps d′ to f′ are repeated by selectinga code value for which no association has been validated, and bysearching for a match between the estimated moment of reception and astored moment of reception of a pulse advanced by an integer number ofsymbol transmission durations. 15: The method as claimed in claim 1,wherein the method is continued until a series of validations isobtained that corresponds to at least half the values contained in thecode. 16: A radio receiver suitable for implementing a synchronizationmethod as claimed in claim
 1. 17: A radio transmission method comprisingthe following steps: coding of the symbols in the form of pulsesdistributed within respective frames, each pulse being transmitted in aslot of the corresponding frame determined by a value of a code storedin a radio transmitter and in at least one radio receiver; transmissionof the pulses by radio channel; and synchronization of at least onereceiver by using a method as claimed in claim
 1. 18: A radiotransmitter suitable for implementing a transmission method as claimedin claim
 17. 19. (canceled) 20: A synchronization device for a radioreceiver, said receiver being suitable for receiving symbols coded inthe form of radio pulses, each transmitted in a predetermined time slotwithin a respective frame, said slot being determined by a valueselected from a set of ordered values forming a code stored in thereceiver, the synchronization device comprising: a—means for storingmoments of reception of pulses; b—means for selecting values from thecode; c—means for calculating at least one second moment of receptionfrom first and second selected code values and a first moment ofreception stored for a first received pulse; d—means for validatingassociations of first and second selected code values with first andsecond received pulses, arranged so as to be activated if a secondcalculated moment of reception matches the moment of reception storedfor the second received pulse; and e—means for determining a moment oftransmission start for a symbol, suitable for determining said startmoment from at least one of the code values whose selection has beenvalidated.